In the production of lubricating oil blend stocks, crude oil is first fractionated in an atmospheric distillation column. The residue of crude oil that is left after the removal of the light ingredients in the crude oil, e.g. straightrun gasoline, kerosene and gas oil, is the basic charge stock for lubricating oil blend stock manufacture. Even though less than 2% of the total crude oil refined in the United States is converted to lubricants, the value of lubricating oil blend stocks makes lubricant manufacture an important part of crude oil refining.
FIG. 1 is a schematic drawing of a conventional facility for making lubricating oil blend stocks. Crude oil is fractionated in atmospheric distillation column 1, and straightrun gasoline, kerosene and gas oil are removed as sidestream cuts from lines A, B and C, respectively. The residue of this crude oil is passed via line 2 into vacuum distillation column 3, where it is vacuum fractionated. The overhead from this fractionation, i.e. gas oil, is taken via line 4 to another part of the refinery, while the liquid bottoms are taken through line 10 to still another part of the refinery. Various lubricating oil blend stock precursors are taken as sidestream cuts and conveyed from vacuum distillation column 3 to storage via lines 5 through 9.
In the next step of the process, the various lubricating oil blend stock precursors are subjected to extraction, dewaxing and clay contacting. This is accomplished by sequentially passing the individual lubricating oil blend stock precursors via line 11 from storage to extraction unit 12, then to dewaxing unit 14 and then to clay contacting unit 16. In extraction unit 12 the lubricating oil blend stock precursors are dearomatized, while in dewaxing unit 14 the wax is separated from the extracted oil and finally in clay contacting unit 16 the oil is decolorized. The resultant lubricating oil fractions are then conducted through line 17 to storage tanks to await blending.
There are several known disadvantages with the lubricating oil blend stocks of the prior art. First, some of the hydrocarbons suitable for making up the lubricating oil blend stock precursors, which should be recovered in sidestreams 5 to 9 of the vacuum distillation column, remain in the vacuum distillation column bottoms and hence are not recovered for use in making lubricating blend stocks. Second, the demand for lubricating blend stocks often exceeds the amount of atmospheric crude oil residue feedstock available.
Accordingly, it is an object of the present invention to provide a modified process for producing lubricating oil blend stocks in which the loss of lubricating oil blend stock precursors from the vacuum tower bottoms is significantly reduced. In addition, it is a further object of the present invention to provide an additional feedstock which is useful in the production of lubricating oil blend stocks.